for those inquiring minds I lifted this from the Jeep online "bible":
Jeep Grand Cherokee WK2 - 2011 Grand Cherokee engines
Under the 6.4 SRT section it says:
Standard Fuel Saver Technology and a new-for-2012 active valve exhaust system allows four-cylinder operation to engage over a wider rpm range, to deliver both improved fuel efficiency - an estimated 13-percent increase on the highway - and an extended range of approximately 450 miles on one tank of gas.
Of course, even SRT owners sometimes appreciate driving more fuel efficiently. For those times, the 2014 Grand Cherokee SRT features an innovative Eco Mode that optimizes the transmission's shift schedule and expands the range of Fuel Saver Technology, which - when conditions are right - deactivates four cylinders to conserve fuel. Eco Mode is available in Selec-Track auto mode. Throttle mapping, transmission shift schedules and Fuel Saving Technology work in combination to achieve maximum fuel efficiency.
I don't know if the Fuel Saver Technology is just another name for MDS but if so, the full MDS explanation is under the 5.7 Hemi section:
About the 5.7L HEMI Multi-Displacement System:
Chrysler Group was the first to offer modern, large-volume vehicles in North America with cylinder deactivation - the HEMI-powered 2005 Chrysler 300C and Dodge Magnum R/T went on sale in the of spring 2004 with the Chrysler Group Multi-Displacement System, or MDS. Chrysler Group was also the first to offer cylinder deactivation in an SUV with the introduction of MDS in the 2005 HEMI-powered Jeep® Grand Cherokee. Chrysler Group is the first to offer MDS in a pickup truck - the 2006 Dodge Ram 1500.
The Chrysler Group MDS is standard equipment with the 5.7L HEMI on seven vehicles: the Chrysler 300C, Dodge Charger R/T, Durango, Magnum R/T, Ram 1500 and Jeep Grand Cherokee and Commander.
With the addition of MDS to HEMI-equipped Dodge Durango and Ram, MDS-equipped vehicles through the end of the 2007 model year will save more than 60 million gallons of fuel each year.
The Chrysler Group MDS (Multi-Displacement System) seamlessly alternates between smooth, high fuel economy four-cylinder mode when less power is needed, and V-8 mode when more power from the 5.7L HEMI® engine is in demand," said Eric Ridenour, Executive Vice President Product Development, Chrysler Group. "This optimizes fuel economy when V-8 power is not needed, without sacrificing vehicle performance.
The MDS was part of the engine's original design," said Bob Lee, Vice President Powertrain Product Team, Chrysler Group. "This resulted in a cylinder-deactivation system that is elegantly simple and completely integrated into the engine design. The benefits are fewer parts, maximum reliability and lower cost.
Some of the significant technologies enabling the Chrysler Group MDS are the speed of electronic controls, the sophistication of the algorithms controlling the systems and the use of Electronic Throttle Control. The HEMI will be able to transition from eight cylinders to four in 40 milliseconds (0.040 seconds).
The HEMI engine with MDS has completed over 6.5 million customer- equivalent miles through Chrysler Group's development and durability testing. It is covered by the 7-Year/70,000-mile Limited Powertrain warranty.
The system deactivates the valve lifters. This keeps the valves in four cylinders closed, and there is no combustion. In addition to stopping combustion, energy is not lost by pumping air through these cylinders. For proper operation, 5w-20 oil must be used in engines with the MDS feature. Failure to do so may result in improper operation of the Multiple Displacement System.
Customers will experience estimated fuel economy gains of up to 20 percent under various driving conditions, and a projected 10 percent aggregate improvement. Improved fuel economy is realized without any change in customer experience -- drivers will receive the benefit without changing their driving habits and without compromising style, comfort or convenience.
MDS components and operation
The Multiple Displacement System (MDS) provides cylinder deactivation during steady speed, low acceleration and shallow grade climbing conditions to increase fuel economy. Both four and eight cylinder configurations have even firing intervals providesmooth operation. Two cylinders on each bank are active when the engine is in four-cylinder mode – every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link. Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring.
Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected.
The Multi Displacement System selectively deactivates cylinders 1,4,6, and 7, to improve fuel economy. It has two modes of operation:
The main components of the Multi Displacement System are:
- 8 cylinders for acceleraton and heavy loads.
- 4 cylinders for cruising and city traffic.
NOTE: Vehicles with the 5.7L Multiple Displacement System must use SAE 5W-20 oil. Failure to do so may result in improper operation of the Multiple Displacement System.
- Unique MDS camshaft.
- Deactivating roller tappets.
- 4 control valves/solenoids.
- control valve/solenoid wiring harness.
- oil temp sensor.
The MDS system is designed to operate at speeds from around 20 mph to 80 mph. Some owners have reported activation at speeds up towards 90 mph. As shown in the chart below, the duty cycle is more active at highway speeds and on level ground. Other parameters that owners have discovered is that the engine operating temperature must be at least 130 degrees and oil pressure above 45 psi. In addition, the transmission must be in 5th gear at speeds above 35-40 mph.
MDS "activation lights" have been added by some owners. This involves wiring up a small light and switch to the No. 4 cylinder MDS solenoid power wire at the PCM pin # 28. While some people can determine the point when MDS is engaged or disengaged by the sound of the exhaust (more notably on aftermarket exhaust systems), the light provides more exacting results as to when the MDS system is on or off.
- Trap an exhaust charge
- Normal combustion event
- Don’t open exhaust valve
- Don’t open intake valve
- Piston is an air spring
- Cylinders deactivated in firing sequence
HEMI MDS duty cycle graph
- Empty the cylinder
- Open exhaust valve
- Open intake valve
- Normal combustion event
- Cylinders reactivated in firing sequence
see below pic
Duty cycle: MDS was engaged for about 17% of city driving and 48% highway for an overall average of 40%
Maximizing MPG with MDS
Just a few simple tips can help owners of Chrysler Group engines with Multi-Displacement System (MDS) get the most fuel mileage possible from their 5.7L HEMI® V8 engine.
The customer does not need to drive in a certain way to realize a fuel economy improvement with MDS, but these driving habits can maximize their fuel savings with this technology.
- Keeping speeds to 65 mph or below – MDS uses four cylinder mode most at these speeds
- Use cruise control – this helps maintain a steady speed, generally allowing the HEMI to run on four cylinders for longer periods
- Accelerate more gradually – the HEMI will provide V8 power whenever it is requested by the driver
- Use a steady throttle whenever possible – this maximizes four cylinder mode